The present invention relates to a novel metallic material suitable for use as material of constituent members of coal gasification processes and other processes in which the constituent members are used in an atmosphere of hot gas containing sulfides. More particularly, the invention is concerned with a sulfidation resisting Cr-Ni-Al-Si alloy which is capable of suppressing high temperature corrosion caused by combustion gases and other product gases.
The oil crisis triggered by the Arab-Israeli war of 1973 has given rise to a demand for developing alternative fuels as substitutes for petroleum. Among these substitute fuels, coal is considered most significant as the basic fuel of the future because of an abundancy of deposits as compared with petroleum. However, since coal is a solid fuel, it is difficult to store and transport as compared with liquid fuel. This in turn has promoted development of techniques for converting coal into a fluid fuel which is easy to store and transport, and also for obtaining clean energy sources through removal of ash, SOx, etc. Typical examples of such techniques are liquefaction and gasification of coal. The gasification of coal is a process in which coal is caused to react with a gasifier such as oxygen, air or steam, thereby obtaining a product gas consisting mainly of hydrogen, carbon monoxide, methane and so forth. Three types of coal gasification processes have been proposed: namely, the fixed bed type, fluidized bed type and entrained bed type. The process type, i.e., the furnace type, and the reaction temperature are selected in accordance with the use of the product gas.
A typical example of a furnace used for the fixed bed type process is a furnace called a "Lurgi furnace." A large scale commercial plant of this type is operating in Sasol in the Union of South Africa. In this process, lumps of coal of sizes ranging between several tens of millimeters and several millimeters are fed from the top of a furnace and are gasified while the coal is held in the form of a bed which is kept stationary. The gasification is effected by the heat which is produced as a result of partial burning of coal with the aid of a gasifier which is supplied from the bottom of the furnace. This process is advantageous in that a high thermal efficiency is obtained by the counter-flow contact between the coal moving downwardly and the gasifier flowing upwardly, but suffers from various disadvantages such as generation of tar in the low temperature region due to a large temperature gradient in the furnace. In addition, this process cannot be applied to the processing of powdered coal and caking coal, and the processing rate is impractically small.
The fluidized bed type process and the entrained bed type process do not suffer from the disadvantage of the fixed bed type process, and are also capable of treating the remnant of crude oil which has to be utilized. For these reasons, intense study and development of these types of coal gasification process are being vigorously undertaken, particularly in U. S. A. and West Germany. In the fluidized bed type process, powdered coal of particle sizes falling within a predetermined range of between several millimeters and several hundreds of microns are charged into a gasification furnace. The powdered coals are fluidized and gasified by a gasifier which is also blown into the furnace. By virtue of the use of powdered coals, this process exhibits a superior heat conduction through convection, so that the reaction takes place uniformly, thus reducing the tendency for tar to be generated as a byproduct. The disadvantage of this type of coal gasification process is that the coal ued in this process has to have such a particle size that adequate fluidity of the coal is maintained.
The entrained bed type process is a process in which pulverized coal of particle sizes ranging between several tens of microns and several hundreds of microns is blown into the furnace from the bottom and is gasified at a high temperature. This process can gasify any type of coal without requiring mechanical stirring or pre-treatment, and is able to gasify the coal almost completely without generation of tar. This process, however, requires pulverization of the coal, and difficulty is experienced in controlling the residence period of the coal in the furnace, as well as in connection with certain problems concerning the system such as facilities for discharge of slag and utilization of sensible heat.
The metallic materials used in coal gasification furnaces are inevitably subjected to high temperature as a result of burning of the coal, unlike the material used in coal liquefaction systems. This imposes a problem of corrosion of the metallic materials by hot gases such as CO, CO.sub.2, H.sub.2, H.sub.2 S and CH.sub.4 which are generated as a result of burning of the coal. In particular, H.sub.2 S at high temperature causes heavy corrosion which is usually referred to as sulfur attack.
In order to put the developed process into practical use on a greater scale, it is necessary to construct a highly reliable plant through development of economical materials or working techniques which enable the constituent elements of the furnace to withstand severe conditions in the gasification process. Thus, the constituent metallic materials used in coal gasification plants are required to withstand the hot corrosive coal gases to which they will be exposed, particularly H.sub.2 S which causes serious sulfur attack.
Among various austenitic steels proposed hitherto, AISI 304 (18Cr-8Ni steel), AISI 316 (18Cr-8NiMo steel), AISI 321 (18Cr-8Ni-Ti steel) and AISI 347 (18Cr-8Ni-Nb steel) are used broadly as the constituent materials for various plants by virtue of their high-temperature strength and workability, as well as low cost and the ease with which they can be manufactured. The use of these austenitic steels is spreading also to the field of piping in nuclear plants and boilers, as a result of improvements in anti-stress corrosion cracking sensitivity through reduction of C content and improvements in anti-steam corrosion properties by refining of the crystal grains. Using these materials for which the ease of production and other properties are known is advantageous from the viewpoint of design, cost and reliability.
These austenitic stainless steels, however, exhibit serious corrosion degradation due to corrosion by gases at high temperatures, particularly grain boundary attack by sulfides.
It has been proposed that the anti-corrosion properties at high temperature may be improved by increasing the Cr content. Examples of materials having increased Cr content are: AISI 309S (21Cr-13Ni steel), AISI 310S (25Cr-20Ni steel), Incoloy 800 (21Cr-32Ni-Ti, Al steel), Inconel 671 (50Cr-50Ni steel) and so forth. These materials have been proposed in view of the fact that inclusion of at least 20 to 25% of Cr is necessary for attaining high corrosion resistance of materials in long use. Attention has been given to these materials because of their ease of manufacture and good workability, but the improvement in their resistance to corrosion by sulfides such as H.sub.2 S is still unsatisfactory due to the fact that the Ni content is necessarily increased in correspondence with the increase in the Cr content in order to maintain the workability and austenitic structure.
Under these circumstances, there is an increasing demand for development of an inexpensive material easy to produce and having high workability, as well as high corrosion resistance equivalent to that of AISI 309S, AISI 310S and Incoloy 800.